Polyurethane-forming compositions are widely used in a variety of commercial, industrial and household applications, such as in automotive clear-coat and seat cushion applications. There is a significant environmental challenge facing the polyurethanes manufacturing industry due to the potential for worker exposure to toxic isocyanates. In order to overcome this challenge, polyurethane systems have been prepared heretofore that employ isocyanates that are pre-reacted with monofunctional reagents to form relatively thermally labile compounds, called blocked isocyanates. Unfortunately, the monofunctional reagents (commonly referred to as "isocyanate blocking agents") are released into the air, upon heating the blocked isocyanate, in order to free-up the functional iscyanate moiety, during curing of the polyurethane-forming composition to prepare the desired polyurethane product. During this curing step, special precautions must be taken to avoid release of these volatile blocking agents into the air (or a waste air stream) in order to protect both workers and the environment.
As an alternative to the use of systems requiring these volatile blocking agents, there are other compounds, namely uretidiones, that are, in effect, "self-blocked", thus providing the desired "latent reactivity" to enable the uretidione linkages to react at the desired time, while avoiding the above-discussed environmental problems associated with the use of volatile blocking agents. The prior art has recognized that there are advantages associated with this latent reactivity, but has failed to make practical application of this concept. By way of illustration, disclosures of the preparation of crosslinkable polyurethane rubbers and elastomers by taking advantage of the latent reactivity of the uretidione linkage can be found in U.S. Pat. Nos. 3,099,642 and 3,248,370, and in British Patent No. 802,189. The processes described in these references involve the combining, at temperatures of less than 100.degree. C., of a relatively high molecular weight (e.g., on the order of from 500 to 3,000 daltons) difunctional resin, a low molecular weight crosslinker reagent, and a uretidione diisocyanate, or a mixture of a uretidione diisocyanate and a monomeric diisocyanate. The preferred uretidiones disclosed in these references are derived from aromatic diisocyanates. The resulting, essentially thermoplastic, formulations contain an excess of isocyanate reactive groups, and these formulations are finally cured to a crosslinked thermoset polymer by treatment at relatively high temperatures for relatively extended periods of time.
Similarly, U.S. Pat. No. 3,793,238 discloses a process for preparing filaments and foils by casting very high molecular weight polyurethanes, derived from high molecular weight polyester diols and aromatic uretidione diisocyanates, from very polar solvents. The final products are crosslinked by exposure to primary or secondary aliphatic diamines or hydrazine or hydrazides.
U.S. Pat. No. 3,524,834 teaches a method of using aromatic uretidione diisocyanates to convert thermoplastic copolymers, derived from mono-ethylenically unsaturated monomers and a minor amount of a co-monomer that contains an amide (--CONH.sub.2) group, into a crosslinked thermoset polymer with heat.
For powder coating systems, the preparation and use of solid, high molecular weight polyuretidione polyurethane curatives, derived from (cyclo)aliphatic diisocyanates and relatively low molecular weight diols, is described in U.S. Pat. Nos.: 4,413,079; 4,463,154; and, 4,483,798; and, in Australian Patent No. A-70,272/94. This last reference teaches the low melt viscosity advantages associated with the use of diols containing carboxylic acid ester and/or carbonate groups in a total amount of at least one percent in the powder coating composition described therein. These diols containing carboxylic acid ester and/or carbonate groups are more expensive than might otherwise be desired.
For solvent-borne coatings systems, polyuretidione based curatives are described in U.S. Pat. No. 3,998,794. These curatives are crosslinkable polyurethane polyurea oligomers containing uretidione groups prepared by chain extending polyisocyanates, containing aliphatically bound uretidione and isocyanate groups, with aliphatic primary and/or secondary diamines. Unfortunately, because the uretidione structures are linked by urea groups, the oligomers that are described in the '794 patent which contain sufficiently high levels of uretidione rings to provide the desired curing also are relatively high in viscosity. Therefore, undesirably high levels of solvents must be used in admixture with these oligomers in order to prepare sprayable coatings formulations.
Thus, while the potential utility and advantages associated with this latent reactivity of uretidiones are known, as illustrated by the disclosures provided in the above-discussed prior art patents, the application of this technology in practical commercial solvent and water borne systems has been lacking heretofore, based upon the knowledge of the present inventors. The reason for this failure in the prior art to find practical commercial application is a lack of understanding being exhibited heretofore regarding how to prepare and use uretidione containing oligomers having the proper balance of uretidione content and chain length in order to achieve materials with useful handling characteristics (notably, viscosity), while providing the needed crosslink density, in order to yield the desired properties in the final, cured polymer.
Accordingly, it would therefore be highly desirable to provide, as a polyurethane-forming cross-linker, a polyuretidione composition having a proper balance of uretidione content and chain length in order to produce a desired combination of properties, including a low solution viscosity, together with a high crosslink density in the final, cured polymer. Such a combination would result in a coating having the desirable performance properties of conventional polyurethane coatings, but without the above-discussed environmental hazards associated with the isocyanates conventionally employed to produce polyurethane coatings. Advantageously, the polyuretidione composition would be useful for producing coatings and elastomers in both solvent-borne and water-borne coatings systems, and would not employ the relatively expensive diols containing carbonate or carboxylic acid ester groups as described in the prior art. The present invention provides one such composition, together with a process for making and using the composition.